1. Field of the Invention
The present invention relates to a satellite broadcast receiving converter, and more particularly, to a fixation and airtight structure for a Low Noise Block Down Converter (that will be referred to as “LNB” hereinafter) that employs a structure where a waveguide connected to a main body portion of the LNB and a waveguide portion of a primary radiator (feedhorn) connected to a tip of the waveguide are separated.
2. Description of the Background Art
Conventional arts will be described with reference to the drawings. FIG. 15 is a schematic diagram of an LNB combined with an antenna system. A radio wave reflected from a parabolic antenna 1 is input to a primary radiator 2 of the LNB.
FIGS. 16 and 17 are cross-sectional views of a general LNB. As shown in these figures, a main body portion (that will be described hereinafter) as well as a waveguide portion 3 and a primary radiator (feedhorn) 2 are integrated in the LNB in most cases and this structure is considered to be ideal in terms of performance. In the main body portion, a circuit board 6 is fixed to a chassis 4 and a frame 5 by a screw 7. A radio wave input from primary radiator 2 is fed to circuit board 6 via waveguide 3 and is output from an F connector 8 after frequency conversion. A cap 9 is fixed to a tip of primary radiator 2 and air tightness is maintained by an O-ring 10.
A recent trend is an increasing number of multi-satellite receiving LNBs as shown in FIG. 18. For example, for a three-satellite receiving converter, a converter has been conventionally configured by arranging three independent LNBs side by side or the like. Recently, however, there is a tendency that LNBs are integrated into a single unit. One of the problems here is a fabrication of an enclosure portion containing a main body portion, a waveguide and a primary radiator. In particular, an enclosure is often made by aluminum die casting, and it is very difficult to stably cast a large and complex-shaped enclosure. Especially in the LNB, it is difficult to keep a casting balance between the waveguide, the primary radiator and the main body portion, and problems such as a reduction in yield, a decrease in dimensional accuracy or a misrun arise. In addition, it is highly likely that the cost of the enclosure portion is increased as a result of a reduction in die life due to an impossible casting condition as well as an increase in weight and degradation in an appearance due to design constraints related to a die structure, and the like. Regarding the material cost, because of the soaring market price, it is also essential in terms of cost and for environmental reasons to reduce the size and weight of the enclosure portion.
As a solution to the above-described problems, it is common to separate the primary radiator portion including the waveguide as a different part, in particular. As a result of the separation, a die structure used for molding the respective parts is simplified and casting is readily performed. Consequently, the productivity is improved and the cost can be reduced. It should be noted that, as a result of the separation, electrical and mechanical performance, air tightness and assemblability should be mainly considered.
The conventional arts of a connecting portion between a waveguide 3 and a feedhorn 2 will be described hereinafter based on FIGS. 19-21. A male thread 11 is formed on an outer circumference of a connecting surface on the chassis side by dicing, and a female thread 12 is formed on the primary radiator side by tapping. Enough electrical contact between a connecting surface on the waveguide 3 side and a connecting surface on the primary radiator 2 side is ensured by screwing-in and tightening, and enough mechanical holding is achieved against a displacement, unscrewing or the like due to mechanical pulling, severe changes in the temperature outside, vibrations on an antenna, or the like. The airtight performance of a joint portion is maintained by applying an adhesive (a sealing agent) 13 to a screwed portion at the time of assembly and further having a fixed O-ring 10 between a tip of primary radiator 2 and an inner surface of a cap 9.
Prior documents disclosing the conventional arts regarding this type of fixation of a waveguide and a feedhorn include Japanese Patent Laying-Open No. 2003-243901, Japanese Patent Laying-Open No. 2004-120348, Japanese Patent Laying-Open No. 08-316701, and the like. Japanese Patent Laying-Open No. 2003-243901 discloses a method of maintaining air tightness by combining a sheet member, a rubber mold and an O-ring as well as by forming a groove portion in a main body portion to hold a sealing agent.
Disclosed in Japanese Patent Laying-Open No. 2004-120348 is a structure that employs a sheet member and a seal or an adhesive as means for maintaining air tightness similarly to that described in the above Japanese Patent Laying-Open No. 2003-243901. Although a method of fixation is not particularly described, a technique for complete mechanical fixation includes a screw-in system as described above or fixation by a screw.
On the other hand, Japanese Patent Laying-Open No. 08-316701 describes the most common structure for maintaining air tightness where the air tightness is maintained by interposing an O-ring in a flange portion fixed by a screw.
In the foregoing conventional examples shown in FIGS. 19-21, although the adhesive is applied to a screw portion (application of a seal) to maintain air tightness, a work often moves (longitudinally, transversely or reversely) at the time of assembly. Furthermore, variations in penetration of the adhesive into the screw portion cause variations in air tightness, and inspections, adjustments or the like are required in some cases. In addition, in the foregoing conventional examples, the adhesive is likely to be squeezed out and a bond needs to be wiped. Hold-and-wait is also required at the time of curing in order to prevent the bond from dripping. Therefore, the productivity is decreased.
Moreover, as for a product requiring the application of coating from the viewpoint of specifications thereof, an influence of the adhesive on the coating needs to be considered. A cleaning process needs to be added and the adhesive needs to be selected in consideration of chemical resistance, heat resistance or the like. In the foregoing conventional examples, a work is coated in a single unit state and assembled, and a flaw or peeling occurs in the coated portion when the coated work is assembled. In addition, the adhesive is squeezed out and wiped, and there are also constraints at the time of curing. Again, the conventional examples are less productive.
As the foregoing conventional examples described in Japanese Patent Laying-Open No. 2003-243901, in the method of maintaining air tightness by combining the sheet member with the rubber mold and the O-ring, a twist due to rotation occurs at the sheet member and the adhesive at the time of screwing-in and fixation. As a result, an adhesive layer is partially destructed or the sheet is deflected. Therefore, air tightness and performance are adversely affected. In addition, it should also be considered that the waveguide portion ideally includes nothing from the viewpoint of performance.
In a case where an O-ring is used, accuracy of a contact surface and an exact crushing rate need to be managed in order to prevent a tear of the O-ring or the poor air tightness of a rough surface portion. As a result, the cost needs to be increased to ensure accuracy of the components. In addition, unless a lubricant such as grease is necessarily used together when the O-ring is compressed by being screwed in, the O-ring is likely to be broken. In particular, as the cross-sectional diameter of the O-ring becomes small, the risk of breakage is significantly increased.
Japanese Patent Laying-Open No. 2003-243901 describes a structure for holding the sealing agent within the groove portion that is formed in a connecting portion between the waveguide on the main body portion side of an LNB and the waveguide portion including the primary radiator (feedhorn). In a structure shown in FIGS. 4 and 6 in the document, the feedhorn side extends so as to cover the waveguide portion of the main body, and air tightness is maintained at a base portion of the main body. The reason why the air tightness is maintained at the base portion may be that, because a groove portion and an outer wall portion need to be formed, the waveguide portion becomes thick, and thus the feedhorn side is extended and fixed.
However, the feedhorn side is extended, so that the component becomes large and a sliding portion of a die becomes long. This is undeniably disadvantageous in terms of castability and the material cost. If the thickness of an extension is reduced, a misrun, the poor air tightness due to a blowhole or a fitting trouble due to deformation is likely to occur. As a result, there is concern that yield of components is worsened. From the viewpoint of the specifications, the longer the waveguide portion is, the more disadvantageous the structure is, In contrast, in a structure shown in FIG. 10 in Japanese Patent Laying-Open No. 2003-243901, the groove portion and the outer wall are provided at the tip. In this case, due to a die structure made in consideration of a process where a molded product is drawn from the die, the waveguide thickness of the groove portion and the outer wall portion must be increased to the base of the main body portion because it is desired that the thickness is at least 0.8 mm or more in consideration of a misrun and the strength of the die from the viewpoint of die cast molding.
Considering the foregoing, it is expected that the thickness is increased by approximately as much as 2 mm including at least the width of the outer wall (0.8 mm) and the width of the groove (0.8 mm for the thickness of a feed insertion portion and 0.2 mm×2 for right and left clearances). In addition to an increase in the material cost, galling is likely to occur because of the uneven thickness (thickness), in particular in the die casting. Furthermore, considering that deformation of the tip due to a deburring process that is one process during the whole process is prevented and a specially-shaped cutting tool is used at the time of threading machining, it is essentially ideal that the structure has a larger dimension.
Japanese Patent Laying-Open No. 2004-120348 describes the structure that employs the sheet member and the seal or the adhesive as means for maintaining air tightness similarly to the above. Although a method of fixation is not particularly described, the screw-in system as described above or fixation by a screw is regarded as a technique for complete mechanical fixation. Similarly, a twist due to rotation occurs at the sheet member and the adhesive in a case of screwing-in. As a result, an adhesive layer is partially destructed or the sheet is deflected. Therefore, there is concern that air tightness and performance are adversely affected. In addition, the waveguide portion ideally includes nothing from the viewpoint of performance.
On the other hand, as described in Japanese Patent Laying-Open No. 08-316701, there is also a method of fixing a flange portion by a screw. In a fixation structure for a waveguide including an LNB, however, it is a problem that the structure becomes large and a die structure becomes complicated. Furthermore, when the flange portion is fixed by the screw, the flange portion needs to be tightened diagonally. Therefore, the structure does not have good workability.